a) Field of the Present Invention
The present invention relates to an optical mouse for moving a cursor displayed on a picture of a display unit such as a CRT (cathode ray tube) or an LCD (liquid crystal display), and more specifically to an improvement of an optical mouse and a resin lens unit applicable to the optical mouse.
b) Related Art
The optical mouse is referred to as a cursor position designating device, which can move a cursor in both X- and Y-axis directions freely on a picture of the display unit such as a CRT.
When this optical mouse is freely moved horizontally and vertically on a mouse pad, the cursor can be moved in both the X- and Y-axis directions on the display unit picture in correspondence with the movement of the optical mouse.
The conventional optical mouse, for example, as disclosed in Japanese Published Examined Patent Application No. 1-39128, provides that the optical mouse includes two line pattern reading apparatuses for reading line patterns formed on a mouse pad, respectively. The line patterns are formed on both surfaces of a transparent substrate of the mouse pad, respectively.
In more detail with reference to FIG. 21, the optical mouse 100 is mainly composed of a housing 112 of reversed-cup shape; a bottom plate 113 for closing the bottom opening of the housing 112; two line reading apparatuses 110X and 110Y mounted on the bottom plate 113; and a circuit board (not shown) for processing electric signals supplied from the line reading apparatuses 110X and 110Y, respectively.
On the other hand, as shown in FIG. 22, the mouse pad 101 is composed of a transparent substrate 111, and two line patterns 102X and 102Y formed by thin parallel lines described on both surfaces of the substrate 111. The substrate 111 is made of a 5 mm thick acrylic resin of roughly A 4 size having a width of about 210 mm and a length of about 300 mm.
The line pattern 102X designates the movement of the cursor in the X-axis direction. The pattern 102X is formed by printing, coating or vacuum depositing a light reflective substance on the right side of the substrate 111. On the other hand, the line pattern 102Y designates the movement of the cursor in the Y-axis direction. The pattern 102Y is formed on the reverse side of the substrate 111 in the same way as with the case of the line pattern 102X so as to intersect the line pattern 102X at an angle of 90 degrees.
The two line reading apparatus 110X and 110Y are fixed on the bottom plate 113 within the housing 112.
Further, in FIG. 21, the reference numeral 114 denotes sliders for stably positioning the optical mouse 100 on the mouse pad 101 so as to be movable on the surface thereof at a right position or attitude.
The location of the optical mouse 100 on the mouse pad 101 can be determined by counting the number of lines of the line pattern 102X in the X-axis direction and further the number of lines of the line pattern 102Y in the Y-axis direction. On the other hand, the cursor displayed on the CRT picture can be determined in correspondence with the position of the optical mouse 100 on the mouse pad 101.
In other words, the movement of the optical mouse 100 on the mouse pad 101 and the movement of the cursor on the CRT picture correspond to each other in one-to-one corresponding relationship. Since the movements of both are two-dimensional, it is possible to determine the X and Y coordinates (a position in the X-axis direction and a position in the Y-axis direction) geometrically. Further, the mutual movement relationship between the two can be processed by a circuit (not shown).
Now, as shown in FIG. 23(A), the X line pattern reading apparatus 110X comprises a light emitting element (LED) 103X for emitting light onto the X line pattern 102X formed on the right side of the transparent substrate 111, a light receiving element 104X for detecting the line pattern 102X, and a convex lens 105X for forming a real image of the X line pattern 102X on the light receiving element (LED) 104X. Further, as shown in FIG. 23(B), the Y line pattern reading apparatus 110Y comprises a light emitting element (LED) 103Y for emitting light onto the Y line pattern 102Y formed on the reverse side of the transparent substrate 111, a light receiving element 104Y for detecting the Y line pattern 102Y, and a convex lens 105Y for forming a real image of the Y line pattern 102Y on the light receiving element (LED 104Y). In the conventional transparent substrate 111 in both the right and reverse surfaces of which X line pattern 102X and Y line pattern 102Y are formed, that is, in the prior art mouse pad 101, the line pattern having a line width of 0.5 mm and a line space of 0.5 mm is formed, respectively, so as to provide a resolving power of 100 CPI (count per inch). The conventional image forming lenses 105X and 105Y used for the reading apparatuses adapted to the mouse pad 101 as described above are of spherical biconvex resin lens (e.g. acrylic resin), which can satisfy the following optical conditions, for instance:
Focal distance: f=5.26 mm
Magnification: .beta.=-4
Number of aperture: NA=0.11
Refractive index (in wave length: 950 nm): n=1.484
Radius of curvature on object side: r.sub.1 =8 mm
Radius of curvature on image plane side: r.sub.2 =-3.28
Central lens thickness: d=3 mm
Lens diameter: .phi.=4 mm
Problems to be Solved by the Invention
FIG. 24 shows the optical paths of the mouse pad reading apparatus having a resolving power 200 CPI (count per inch) twice as large as the conventional mouse pad resolving power, to which the two image forming lenses 105X and 105Y satisfying the above-mentioned optical conditions are applied. In the drawing, the reference numeral 102 (102X and 102Y) denotes a line on the mouse pad; 105 denotes a spherical lens satisfying the above-mentioned optical conditions; and 106 denotes a formed image. Further, in FIG. 24, light beams directed from the line 102 (102X and 102Y) to the light receiving elements 104X and 104Y are shown under consideration of the spherical aberration. As also shown in FIG. 23, the light receiving elements 104X and 104Y are required to be composed of a pair of light receiving elements 104X and 104Y or light receiving portions 104a and 104b in order to obtain a phase difference signal for detecting the movement direction (positive or negative direction) of the optical mouse. The phase difference signal is explained as follows: when the optical mouse is moved, the light energies inputted to the two light receiving elements 104X and 104Y, respectively, vary with respect to time, so that the wave forms of the signals outputted from the two light receiving elements 104X and 104Y also vary. The change in waveform between the two output signals with respect to time can be represented as a phase difference and detected as a phase different signal. Therefore, the movement direction of the optical mouse can be determined on the basis of the phase difference. Accordingly, since the light receiving elements 104X and 104Y are required to be composed of a pair of light receiving elements 104X and 104Y or light receiving portions 104a and 104B, it is preferable that the area of the image is twice as large as the area of the light receiving elements 104X and 104Y or the light receiving portions 104a and 104b. Here, when the size of the light receiving element 104X or 104Y or the light receiving portion 104a or 104b is 0.5 mm .times.0.5 mm and the resolving power is 200 CPI, since the line width is 0.25 mm, in order to obtain an image of 1 mm in size, it is necessary to adjust the positions of the line 102 (102X and 102Y), the lenses 105 (105X and 105Y) and the light receiving elements 104X and 104Y, respectively, so that the magnification becomes 4 times.
FIG. 25 is a series of diagrams showing the various aberrations of the lens which satisfies the above-mentioned optical conditions. In the drawing, the ordinate in the spherical aberration, the astigmatism, or the distortion aberration denotes the height of the incident light (the incident height on the pupil of the eye) h, respectively, where h is 2 mm in full scale. Further, in the coma aberration diagrams, y=0 indicates the coma aberration on the optical axis (height h=0 mm) on the image plane (the surface of the light receiving element); y=0.5 mm indicates the coma aberration at a height (h=0.5 mm) in the direction perpendicular to the optical axis on the surface of the light receiving element; and y=1.0 mm indicates the coma aberration at a height (h=1.0 mm) in the direction perpendicular to the optical axis on the surface of the light receiving element. As clearly understood with reference to FIG. 25, since the lens is a spherical lens, the spherical aberration and coma aberration are large. In order to achieve the positional relationship of magnification of 4 (.beta.=-4), it is originally necessary to position the light receiving element 104 (104X and 104Y) at the image plane 106 shown in FIG. 21. However, since the spherical aberration is large and thereby an unclear line image appears on the image plane 106, in practice, it is necessary to locate the light receiving element 104 (104X and 104Y) at an optimum position within the adjustment range determined away from the image plane 106 toward the lens. In this case, since the above-mentioned relationship of (.beta.=-4) cannot be satisfied within the adjustment range for locating the light receiving element, the position of the light receiving element 104 must be determined so that the most clear image can be formed at roughly the magnification of 4 times by additionally adjusting the interval between the line 102 (102X and 102Y) and the lens 105 (105X and 105Y). When the optimum position can be determined, if the resolving power is 100 CPI and the line width and the light receiving element are both large, it is possible to barely obtain a photocurrent signal (a signal outputted from the light receiving element for transducing the optical energy (signal) into an electrical signal) and a phase difference signal, as far as the line image is large and therefore the photocurrent is large and further the error in the position adjustment is small. However, when the resolving power is high i.e., such as 200 CPI or more, and further the line width and the light receiving element are both small, even if the above-mentioned adjustment is made as finely as possible, it is impossible to obtain the photocurrent signal; that is, to read the mouse pad. This is because there exists a limit in the detection sensitivity caused by the unclear image resulting from the spherical aberration of the lens. In other words, there exists a problem in that it is necessary to continuously increase the light emission intensity of the light emitting element because of the unclear image, thus increasing the power consumption.